There is a cavity in my gum in my lower teeth below my tounge. it's pains when i have citrus fruits or flex my tongue. it's small white cavity .

Cavity?: A cavity is a hole in something. Sounds like you have a an issue with your glands/ducts in the floor of your mouth, like a salivary stone, but your description is inconsistent. Is it in your gums? In your lower teeth? Or under your tongue? Post a photo or see your dentist for more help.

Ulcer: You may have an ulcer in that area. If it does not resolve in 2 weeks time see your physician to have it looked at.